Thin-film transistor (TFT) devices are widely used in switching or driver circuitry for electro-optical arrays and display panels. TFT devices are conventionally fabricated on rigid substrates, typically glass or silicon, using a well-known sequence of deposition, patterning and etching steps. For example, amorphous silicon TFT devices require deposition, patterning, and etching of metals, such as aluminum, chromium or molybdenum; of amorphous silicon semiconductors; and of insulators, such as SiO2 or Si3N4, onto a substrate. The semiconductor thin film is formed in layers having typical thicknesses ranging from several nm to several hundred nm, with intermediary layers having thicknesses on the order of a few microns, and may be formed over an insulating surface that lies atop the rigid substrate.
The requirement for a rigid substrate has been based largely on the demands of the fabrication process itself. Thermal characteristics are of particular importance, since TFT devices are fabricated at relatively high temperatures. Thus, the range of substrate materials that have been used successfully is somewhat limited, generally to glass, quartz, or other rigid, silicon-based materials.
TFT devices can be formed on some types of metal foil and plastic substrates, allowing some measure of flexibility in their fabrication. However, problems such as chemical incompatibility between the substrate and TFT materials, thermal expansion mismatch between substrate and device layers, planarity and surface morphology, and capacitive coupling or possible shorting make metal foil substrates more difficult to employ in many applications.
The fabrication process for the TFT may require temperatures in the range of 200-300 degrees C. or higher, including temperatures at levels where many types of plastic substrates would be unusable. Thus, it is widely held, as is stated in U.S. Pat. No. 7,045,442 (Maruyama et al.), that a TFT cannot be directly formed on a plastic substrate. In order to provide the benefits of TFT devices mounted on a plastic substrate, the Maruyama et al. '442 disclosure describes a method that forms the TFT on a release layer that is initially attached to a carrier substrate. Once the TFT circuitry is fabricated, the release layer is then separated from its carrier substrate and can be laminated onto a lighter and more flexible plastic material.
As one alternative solution, U.S. Pat. No. 6,492,026 (Graff et al.) discloses the use of flexible plastic substrates having relatively high glass transition temperatures Tg, typically above 120 degrees C. However, the capability for these substrates to withstand conventional TFT fabrication temperatures much above this range is questionable. Moreover, in order to use these plastics, considerable effort is expended in protecting the substrate and the device(s) formed from scratch damage and moisture permeation, such as using multiple barrier layers.
Another alternative solution is described in U.S. Pat. No. 6,680,485 (Carey et al.) In the method described in the Carey et al. '485 disclosure, energy from a pulsed laser source is used to form amorphous and polycrystalline channel silicon TFTs onto low-temperature plastic substrates. The conventional, low-temperature plastic substrates for which this method is described include polyethylene terephthalate (PET), polyethersulfone (PES), and high density polyethylene (HDPE), for example.
Similarly, U.S. Pat. No. 6,762,124 (Kian et al.) discloses a process using an excimer laser to ablate a material through a mask to form a patterned conductor or semiconductor material for TFT formation onto a substrate. In the Kian et al. '124 disclosure, the substrate that is used is a composite, “glass replacement” material that may have a flexible or rigid plastic material supplemented with one or more barrier and protective layers.
Although these and similar solutions have been proposed for forming TFT components on flexible substrates, drawbacks remain. Lamination of a release layer that is populated with TFT devices, as described in Maruyama et al. '442 requires additional fabrication steps and materials and presents inherent alignment difficulties. The use of high-performance plastics, such as that of the Graff et al. '026 disclosure, still leaves thermal expansion (expressed as Coefficient of Thermal Expansion, CTE) difficulties and requires additional layers and processes in order to protect the plastic. The excimer laser solutions proposed in the Carey et al. '485 and Kian et al. '124 disclosures do not provide the full breadth of capabilities of more conventional TFT fabrication techniques and thus have limited utility. None of these disclosures provides a flexible substrate that truly serves to replace glass or other silicon-based substrate, since the TFT must be formed either on a release layer or on some intermediate layer that must be formed on top of the flexible substrate.
TFT fabrication onto flexible substrates generally requires that the substrate be held on a carrier of some type during the various stages of layer deposition. One of the more important functions of such a carrier is providing dimensional stability to the flexible substrate. Thus, for example, a rigid glass carrier is conventionally provided. As described in Japanese Patent Publication Number JP 7-325297 A2 (Ichikawa), TFT devices can be formed onto a plastic substrate temporarily held to a glass carrier by means of an adhesive layer.
The use of a glass carrier imposes some constraints on the types of flexible substrate materials that can be used. Some types of plastics are compatible with the use of a glass substrate, but can be impractical because they exhibit transition Tg temperatures near the region of temperatures used for deposition. Thus, plastic substrates can tend to soften somewhat, allowing expansion during a fabrication cycle. Metals do not have this disadvantage. However, metallic materials are not as dimensionally “forgiving” with change in temperature. A significant difference in coefficient of thermal expansion (CTE) between metals and glass results in excessive stress that can shatter glass or can cause a metal substrate to release from a glass carrier prematurely, losing its dimensional stability.
Thus, it can be seen that although there has been great interest in developing and expanding the use of both plastics and metals as flexible substrates, compatibility with a conventional glass carrier imposes some constraints on substrate material type. For this reason, there is a need for carrier materials, other than glass, that can be employed for TFT fabrication onto flexible substrates.